Anisotropic microporous syndiotactic polystyrene membranes and a process for preparing the same

ABSTRACT

This invention relates to semi-permeable anisotropic syndiotatic polystyrene microporous membranes and a process for preparing such membranes.

BACKGROUND OF THE INVENTION

This invention relates to anisotropic microporous membranes comprised of syndiotactic polystyrene and a process for preparing such membranes.

Microporous membranes have long been used in the liquid membrane separation processes of ultrafiltration, microfiltration, membrane distillation, and membrane stripping. Ultrafiltration and microfiltration are pressure driven filtration processes using microporous membranes in which particles or solutes are separated from solutions based on differences in particle size, particle shape, and/or molecular weight. Membrane distillation and membrane stripping are separation processes using microporous membranes in which certain components of the liquid to be treated which are more volatile permeate through the membrane as vapor more rapidly than components which are less volatile due to differences in chemical potential across the membrane. In membrane distillation, the permeated components are condensed on the permeate side of the membrane and removed as liquid, while in membrane stripping the permeated components are removed from the permeate side of the membrane as vapor.

Such microporous membranes may be isotropic or anisotropic (asymmetric). Isotropic microporous membranes possess a morphology in which the pore size within the membrane is substantially uniform throughout the membrane. Anisotropic (asymmetric) microporous membranes possess a morphology in which a pore size gradient exists across the membrane; that is, the membrane morphology varies from highly porous, larger pores at one membrane surface to less porous, smaller spores at the other membrane surface. Such anisotropic membranes thus possess a microporous "skin" of smaller pores. The term asymmetric is often used interchangeably with the term anisotropic.

In the past, such microporous membranes have been fabricated from aliphatic polyolefins such as polyethylene and polypropylene, or from high performance polymers such as sulfonated polyetheretherketone.

However, the aliphatic polyolefin polymers presently used, while inexpensive and easy to process, exhibit relatively low heat distortion temperatures. The high performance polymers, such as sulfonated polyetheretherketone, are derived from polymers which are difficult to process and quite expensive.

What is needed are anisotropic microporous membranes useful for ultrafiltration, microfiltration, membrane distillation, and/or membrane stripping which possess good solvent resistance and heat distortion temperatures, are easily processed, and are prepared from low-cost materials.

SUMMARY OF THE INVENTION

The invention is a semi-permeable membrane comprising a thin anisotropic syndiotactic polystyrene microporous membrane.

In another aspect, the invention is a process for preparing a semi-permeable anisotropic syndiotactic polystyrene microporous membrane comprising the steps of:

A. forming a mixture comprising:

(i) syndiotactic polystyrene,

(ii) at least one protic solvent for the syndiotactic polystyrene,

(iii) optionally at least one non-solvent for the syndiotactic polystyrene:

B. heating the mixture to a temperature under conditions such that a homogeneous fluid is formed which possesses sufficient viscosity to be formed into a membrane:

C. extruding or casting the homogeneous fluid into a membrane:

D. quenching or coagulating the membrane by passing the membrane through one or more zones under conditions such that the membrane solidifies;

E. simultaneously or consecutively leaching the membrane by passing the membrane through one or more zones under conditions such that at least a substantial portion of the solvent and optional non-solvent for the syndiotactic polystyrene is removed from the membrane; and

F. optionally drawing the membrane before, during, and/or after leaching at a temperature at or above ambient temperature and below the melting point of the syndiotactic polystyrene or the depressed melting point of the polystyrene/solvent/optional non-solvent mixture to elongate the membrane and to induce orientation of the syndiotactic polystyrene in the membrane:

wherein the semi-permeable membrane so formed possesses anisotropic microporous structure.

The semi-permeable membranes so formed are useful in ultrafiltration, microfiltration, membrane distillation, and/or membrane stripping, or as supports for composite gas or liquid separation membranes. The semi-permeable membranes of this invention possess good solvent resistance and heat distortion properties. The semi-permeable membranes may be prepared with relative ease and low cost.

DETAILED DESCRIPTION OF THE INVENTION

The membranes of this invention are prepared from syndiotactic polystyrene. Polystyrene is represented by the formula: ##STR1## in which the phenyl group is pendant to the polymer backbone. Polystyrene may be isotactic, syndiotactic, or atactic, depending upon the positional relationship of the phenyl groups to the polymer backbone. Syndiotactic polystyrene is polystyrene wherein the phenyl groups which are pendent from the polymer backbone alternate with respect to which side of the polymer backbone the phenyl group is pendent. In other words, every other phenyl group is on the same side of the polymer backbone. Isotactic polystyrene has all of the phenyl groups on the same side of the polymer backbone. Standard polystyrene is referred to as atactic, meaning it has no stereoregularity; the placement of the phenyl groups with respect to each side of the polymer backbone is random, irregular, and follows no pattern. For further definition and description of stereoregular polymers, see Leo Mandelkern, An Introduction to Macromolecules, 2nd edition, Springer-Verlag, New York, N.Y., 1983, pp. 49-51, the relevant portions incorporated herein by reference.

The properties of polystyrene vary according to the tacticity of the polystyrene, that is, the positional relationship of the pendent phenyl groups to the polymer backbone. Atactic polystyrene is generally unsuitable for membrane formation because of its lack of crystallinity and poor solvent resistance. Isotactic polystyrene possesses improved solvent and temperature resistance over atactic polystyrene. However, isotactic polystyrene is somewhat slow to produce suitable gels from its solutions at ambient conditions necessary to form membranes. Isotactic polystyrene crystallizes very slowly to form crystalline species that melt at about 190-240° C.; nucleating agents often must be added to facilitate its crystallization. Syndiotactic polystyrene exhibits excellent heat distortion temperature properties. Syndiotactic polystyrene rapidly crystallizes to form crystalline species that melt at about 269-276° C.; nucleating agents are generally not needed to facilitate its crystallization.

Syndiotactic polystyrene may be prepared by methods well known in the art. See Japanese Patent 104818 (1987); Ishihara et al., U.S. Pat. No. 4,680,353; and Ishihara, Macromolecules, 19 (9), 2464 (1986); the relevant portions incorporated herein by reference. The term syndiotactic polystyrene includes polystyrene in which syndiotactic stereoregularity predominates. Syndiotactic polystyrene, as defined herein, preferably possesses a degree of syndiotactic tacticity of at least about 55 percent, more preferably of at least about 70 percent, even more preferably of at least about 80 percent. The percent tacticity of syndiotactic polystyrene may be determined from nuclear magnetic resonance spectra and X-ray diffraction patterns by methods known in the art. See "Stereoregular Linear Polymers" and "Stereoregular Styrene Polymers", Encyclopedia of Polymer Science and Engineering, Vol. 15, John Wiley & Sons, New York, New York, 1989, pp. 632-763 and "Nuclear Magnetic Resonance", Encyclopedia of Polymer Science and Engineering, Vol. 10, John Wiley & Sons, New York, New York, 1987, pp. 254-327; the relevant portions incorporated herein by reference. The syndiotactic polystyrene of this invention may optionally contain small amounts of inert compounds, inhibitors, stabilizers, or other additives which do not adversely affect the membrane-forming ability of the syndiotactic polystyrene/solvent/optional non-solvent mixture. Preferably the presence of these minor impurities in the syndiotactic polystyrene is less than about 10 weight percent, more preferably less than about 5 weight percent, most preferably less than about 1 weight percent.

The invention comprises a semi-permeable anisotropic syndiotactic polystyrene microporous membrane and a process for preparing the same.

Solvents useful in this invention are those compounds which are a liquid at membrane fabrication temperatures and which dissolve a sufficient amount of the syndiotactic polystyrene to result in a solution viscous enough to form membranes. Solvents dissolve at the membrane fabrication temperature, at least about 5 weight percent of the syndiotactic polystyrene preferably at least about 10 weight percent, more preferably at least about 25 weight percent, even more preferably at least about 50 weight percent. The boiling points of the solvents useful in this invention are preferably above the membrane fabrication temperature so that a significant portion of the solvent is not flashed off during the extrusion or casting step but is instead retained in the membrane until the quenching/coagulating and/or leaching steps. The solvents preferably possess a boiling point of at least about 130° C., more preferably of at least about 140° C. The solvents useful in this invention may be protic or aprotic. A protic solvent as used herein refers to a solvent which is capable of dissolving the polystyrene, in which one of the steps of dissolution involves the generation of protons or hydrogen ions. An aprotic solvent as used herein refers to a solvent which is capable of dissolving the polystyrene, in which one of the steps of dissolution does not involve the generation of protons or hydrogen ions.

Preferred solvents for syndiotactic polystyrene include substituted benzenes of the formulas: ##STR2## wherein R¹ is hydrogen, alkyl, cycloalkyl, halo, or nitro;

R² is alkyl;

R³ is alkyl, aryl, carboxyaryl, or alkoxy;

a is an integer of from 1 to 3;

b is an integer of from 0 to 3; and

c is an integer of from 1 to 2.

Other preferred solvents include alkyl, cycloalkyl, aryl, or aralkyl substituted pyrrolidinones; chloronaphthalenes; hydrogenated and partially hydrogenated naphthalenes; aryl substituted phenols; ethers of the formula: ##STR3## wherein R⁴ is alkyl, cycloalkyl, or aryl; diphenyl sulfone; benzyl alcohol; bisphenol A; caprolactam; caprolactone; alkyl aliphatic esters containing a total of from 7 to 20 carbon atoms; alkyl aryl substituted formamides; dicyclohexyl; terphenyls; partially hydrogenated terphenyls; and mixtures of terphenyls and quaterphenyls.

Preferred substituted benzene solvents include o-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, xylene, nitrobenzene, acetophenone, methyl benzoate, ethyl benzoate, diphenyl phthalate, benzil, methyl salicylate, benzophenone, cyclohexyl benzene, n-butylbenzene, n-propylbenzene, phenol, and dimethyl phthalate. Examples of preferred ethers include phenetole (phenyl ethyl ether), diphenyl ether, and anisole. Examples of preferred pyrrolidinone solvents include 1-benzyl-2-pyrrolidinone, 1-cyclohexyl-2-pyrrolidinone, 1-ethyl-2-pyrrolidinone, 1-methyl-2-pyrrolidinone, and 1-phenyl-2-pyrrolidinone. More preferred pyrrolidinone solvents include the alkyl and cycloalkyl substituted pyrrolidinones. Even more preferred pyrrolidinone solvents include 1-cyclohexyl-2pyrrolidinone, 1-ethyl-2-pyrrolidinone, and 1-methyl-2pyrrolidinone. Preferred ether solvents include anisole and diphenyl ether. Preferred hydrogenated naphthalene solvents include decahydronaphthalene (decalin) and tetrahydronaphthalene (tetralin). Examples of terphenyls and partially hydrogenated terphenyls preferred include partially hydrogenated terphenyls, available from Monsanto under the tradename Therminol® 66; mixed terphenyls and quaterphenyls, available from Monsanto under the tradename Therminol® 75; and mixed terphenyls available from Monsanto under the Santowax® R tradename. More preferred aliphatic esters are those methyl aliphatic esters with a total of from 10 to 14 carbon atoms, with methyl laurate being most preferred.

More preferred solvents include 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, 1-ethyl-2-pyrrolidinone, 1-methyl-2-pyrrolidinone, 1-cyclohexyl-2-pyrrolidinone, acetophenone, anisole, benzil, benzophenone, benzyl alcohol, bisphenol A, caprolactam, caprolactone, decahydronaphthalene, tetrahydronaphthalene, diphenyl ether, ethyl benzoate, methyl salicylate, dichlorobenzene, mixed terphenyls, and partially hydrogenated terphenyls.

Water miscible solvents are a preferred class of solvents which include 1-cyclohexyl-2-pyrrolidinone, 1-methyl-2-pyrrolidinone, caprolactam, caprolactone, N,N-diphenylformamide, and sulfolane.

Alkali miscible solvents are a preferred class of solvents which include alcohols and phenols.

Protic solvents preferred for use in this invention include 4-phenylphenol, benzyl alcohol, bisphenol A, caprolactam, phenetole, and phenol.

Aprotic solvents preferred for use in this invention include o-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, xylene, nitrobenzene, acetophenone, methyl benzoate, ethyl benzoate, diphenyl phthalate, benzil, methyl salicylate, benzophenone, cyclohexyl benzene, n-butylbenzene, n-propylbenzene, anisole, 1-benzyl-2-pyrrolidinone, 1-cyclohexyl-2-pyrrolidinone, 1-ethyl-2-pyrrolidinone, 1-methyl-2-pyrrolidinone, 1-phenyl-2-pyrrolidinone, decahydronaphthalene (decalin), tetrahydronaphthalene (tetralin), methyl laurate, and caprolactone.

The solvents useful in this invention may optionally contain small amounts of inert compounds, inhibitors, stabilizers, or other additives which do not adversely affect the membrane-forming ability of the syndiotactic polystyrene/solvent/optional non-solvent mixture. Preferably the presence of these minor impurities in the solvent is less than about 10 weight percent, more preferably less than about 5 weight percent, most preferably less than about 1 weight percent.

The optional non-solvents useful in this invention are those compounds which are a liquid at membrane fabrication temperatures and which dissolve less than about 5 weight percent of the syndiotactic polystyrene at the membrane fabrication temperature. The boiling points of the non-solvents useful in this invention are preferably above the membrane fabrication temperature so that a significant portion of the nonsolvent is not flashed off during the extrusion or casting step but is instead retained in the membrane until the quenching/coagulating and/or leaching steps. The non-solvents preferably possess a boiling point of at least about 100° C., more preferably of at least about 120° C.

Non-solvents preferred for use in this invention include dimethylsulfoxide, ethylene carbonate, methyl caproate, methyl caprylate, methyl enanthate, methyl valerate, mineral oil, and paraffin oil.

The non-solvents useful in this invention may optionally contain small amounts of inert compounds, inhibitors, stabilizers, or other additives which do not adversely affect the membrane-forming ability of the syndiotactic polystyrene/solvent/optional non-solvent mixture. Preferably the presence of these minor impurities in the non-solvent is less than about 10 weight percent, more preferably less than about 5 weight percent, most preferably less than about 1 weight percent.

The concentration of syndiotactic polystyrene in the casting or extrusion mixture may vary. The concentration of the syndiotactic polystyrene in the mixture is dependent upon the solvent and optional non-solvent, the molecular weight of the syndiotactic polystyrene, and the viscosity of the mixture. The syndiotactic polystyrene molecular weight should be sufficient such that membranes with reasonable physical integrity can be formed. The weight-average molecular weight (Mw) of the syndiotactic polystyrene is preferably at least about 200,000 daltons, more preferably at least about 400,000 daltons. The preferred upper limit on the weight-average molecular weight is about 5,000,000 daltons, with about 3,000,000 daltons being more preferred. The mixture must possess sufficient viscosity to enable casting or extruding the mixture into a membrane. If the viscosity is too low, the membranes will lack physical integrity; if the viscosity is too high, the mixture cannot be formed into membranes. Preferably the lower limit on viscosity at the membrane casting or extrusion step is about 20 poise, more preferably about 40 poise. The upper limit on viscosity at the membrane casting or extrusion step is preferably about 1,000,000 poise, more preferably about 500,000 poise, most preferably about 100,000 poise. Preferably, the mixture contains between about 5 and about 90 weight percent syndiotactic polystyrene, more preferably between about 10 and about 80 weight percent syndiotactic polystyrene, even more preferably between about 15 and about 70 weight percent syndiotactic polystyrene. The amount of optional non-solvent used in the mixture is such that the solvent and non-solvent together dissolve at least about 5 weight percent of the syndiotactic polystyrene present at the membrane fabrication temperature, preferably at least about 10 weight percent, more preferably at least about 5 weight percent. Preferably the amount of optional non-solvent in the mixture is less than about 20 weight percent, more preferably less than about 15 weight percent.

The membranes of this invention may be prepared by solution casting or extrusion. In the solution casting process, the syndiotactic polystyrene is contacted with at least one solvent and optionally at least one non-solvent for the syndiotactic polystyrene at elevated temperatures. The elevated temperature at which the mixture is contacted is that temperature at which the mixture is a homogeneous fluid, and below that temperature at which the syndiotactic polystyrene degrades and below that temperature at which the solvent and optional non-solvent boils. The upper temperature limit is preferably below about 325° C., more preferably below about 300° C. The minimum temperature limit is preferably at least about 25° C. The contacting takes place with adequate mixing or agitation to ensure a homogeneous solution.

In the case of casting, a membrane may be cast into flat sheet form by pouring the mixture onto a smooth support surface and drawing down the mixture to an appropriate thickness with a suitable tool such as a doctor blade or casting bar. Alternately, the mixture may be cast in a continuous process by casting the solution onto endless belts or rotating drums. The casting surface may be such that the membrane may thereafter be readily separated from the surface. For example, the membrane may be cast onto a support having a low surface energy, such as silicone, coated glass, Teflon, or metal, or a surface to which the membrane will not adhere. The mixture may also be cast onto the surface of a liquid with which the syndiotactic polystyrene is immiscible, such as water or mercury. Alternately, the mixture may be cast onto a support surface which may thereafter be dissolved away from the finished membrane. The membrane may also be cast onto a porous support surface. The cast membrane is thereafter subsequently quenched or coagulated, leached, and optionally drawn as described hereinafter for anisotropic microporous membranes formed by the extrusion process.

Extrusion is the preferred process for the fabrication of flat sheet, tubular, or hollow fiber membranes. In the case of extrusion, the components of the extrusion mixture may be combined prior to extrusion by mixing in any convenient manner with conventional mixing equipment, as for example, in a Hobart mixer or a resin kettle. Alternately, the extrusion mixture may be homogenized by extruding the mixture through a twin screw extruder, cooling the extrudate, and grinding or pelletizing the extrudate to a particle size readily fed to a single or twin screw extruder. The components of the extrusion mixture may also be combined directly in a melt-pot or twin screw extruder and extruded into membranes in a single step.

The mixture of syndiotactic polystyrene/solvent/optional non-solvent is heated to a temperature at which the mixture becomes a homogeneous fluid. The homogeneous fluid is then extruded through a sheet, hollow tube, or hollow fiber die (spinnerette). Hollow fiber spinnerettes are typically multi-holed and thus produce a tow of multiple hollow fibers. The hollow fiber spinnerettes include a means for supplying fluid to the core of the extrudate. The core fluid is used to prevent collapse of the hollow fibers as they exit the spinnerette. The core fluid may be a gas such as nitrogen, air, carbon dioxide, or other inert gas, or a liquid with which the syndiotactic polystyrene is substantially immiscible, such as water.

Following casting or extruding, the membrane is passed through at least one quench or coagulation zone under conditions such that the membrane solidifies. The environment of the quench or coagulation zone may be a gas, a liquid, or a combination thereof. Within the quench or coagulation zone, the membrane is subjected to cooling and/or coagulation to cause gelation and solidification of the membrane. In one preferred embodiment, the membranes are first quenched in a gas, preferably air. Within the gas quench zone, the membranes gel and solidify. A portion of the solvent and optional non-solvent may evaporate and the membrane pore structure may begin to form. The temperature of the gas quench zone is such that the membrane gels and solidifies at a reasonable rate. The temperature of the gas quench zone is preferably at least about 0° C., more preferably at least about 15° C. The temperature of the gas quench zone is preferably less than about 100° C., more preferably less than about 50° C. Ambient temperatures are particularly convenient and suitable for the gas quench zone. Shrouds may be used to help control gas flow rates and temperatures in the gas quench zone. The residence time in the gas quench zone is such that the membrane gels and solidifies. The residence time in the gas quench zone is preferably at least about 0.01 seconds, more preferably at least about 0.05 seconds. The residence time in the gas quench zone is preferably less than about 10 seconds, more preferably less than about 8 seconds.

Following or instead of the gas quench, the membranes may be quenched or coagulated in a liquid which does not dissolve the syndiotactic polystyrene. The primary function of the quench or coagulation liquid may be to provide a sufficient heat transfer media to solidify the membrane. However, the quench or coagulation liquid may optionally also be a solvent for the syndiotactic polystyrene solvent and optional non-solvent so as to enable removal of at least a portion of the syndiotactic polystyrene solvent and optional non-solvent from the membrane during the quenching and/or coagulation step. Preferred liquid quench or coagulation zone materials include water, lower alcohols, phenols, halogenated hydrocarbons, and perhalogenated carbon compounds. Perhalogenated carbon compounds are materials with a carbon backbone wherein all of the hydrogen atoms have been replaced by halogen atoms. More preferred liquid quench or coagulation materials include water, chlorinated hydrocarbons, and lower alcohols, with lower alcohols being most preferred. Preferred lower alcohols are C₁₋₄ alcohols The lower limit on the liquid quench or coagulation zone is that temperature at which the liquid material freezes. The lower temperature limit on the liquid quench or coagulation zone is preferably at least about 0° C. The upper limit on the liquid quench or coagulation zone is either the boiling points of the syndiotactic polystyrene solvent and optional non-solvent or that temperature above which the membrane does not undergo solidification when in contact with the liquid quench or coagulation material, whichever is lower. The upper temperature limit on the liquid quench or coagulation zone is preferably less than about 100° C., more preferably less than about 50° C. Ambient temperatures are suitable and convenient. The residence time in the liquid quench or coagulation zone is such that the membrane gels and solidifies. The residence time in the liquid quench or coagulation zone is preferably at least about 0.01 seconds, more preferably at least about 0.05 seconds. The residence time in the liquid quench or coagulation zone is preferably less than about 200 seconds, more preferably less than about 100 seconds.

Following quenching and/or coagulation, the membrane is passed through at least one leach zone under conditions such that at least a substantial portion of the solvent and optional non-solvent for the syndiotactic polystyrene is removed from the membrane. The leach zone material is a solvent for the syndiotactic polystyrene solvent and optional nonsolvent but does not dissolve the syndiotactic polystyrene. The materials which may be used in the leach zone are the same as the materials which may be used in the liquid quench zone. The minimum temperature of the leach zone is that temperature at which the syndiotactic polystyrene solvent and optional nonsolvent are removed from the membrane at a reasonable rate. The minimum temperature of the leach zone is preferably at least about 0° C., more preferably at least about 15° C. The maximum temperature of the leach zone is below that temperature at which membrane integrity is adversely affected. The maximum temperature of the leach zone is preferably less than about 150° C., more preferably less than about 100° C. The residence time in the leach zone is that which is sufficient to remove at least a portion of the syndiotactic polystyrene solvent and optional nonsolvent from the membrane. Preferably, a substantial portion of the remaining syndiotactic polystyrene solvent and optional nonsolvent is removed from the membrane in the leach zone. The residence time in the leach zone is preferably between about 1 second and about 24 hours, more preferably between about 30 seconds and about 8 hours. The leach may be performed as a continuous or batch process. The residence time is dependent upon the particular solvent and optional nonsolvent, the membrane size and thickness, and the kinetics for removing the solvent and optional nonsolvent from the membrane.

Before, during, and/or after leaching, the membranes may be drawn down or elongated to the appropriate size and thickness. Drawing down or elongating means the membranes are stretched such that the length of the membrane is longer and the diameter smaller at the end of the drawing or elongation process. Drawing increases the mechanical strength of the membrane by inducing orientation in the membrane. The draw temperature is dependent upon whether the membrane contains solvent and optional nonsolvent at the time of drawing For substantially solvent and optional nonsolvent free membranes, the membrane is heated to a temperature between the glass transition temperature of syndiotactic polystyrene and the melting point of syndiotactic polystyrene, with preferred lower temperatures being at least about 90° C., more preferably at least about 100° C., and with preferred upper temperatures being less than about 280° C., more preferably less than about 270° C. For membranes containing solvent and optional nonsolvent, the membrane is heated to a temperature between ambient temperature and the melting point of syndiotactic polystyrene or the depressed melting point of the polystyrene/solvent/optional nonsolvent mixture, with preferred lower temperatures being at least about 10° C., more preferably at least about 25° C., and preferred upper temperatures being less than about 10° C. below the depressed melting point. The membrane is drawn by stretching the membrane under tension. Flat sheet membranes may be uniaxially or biaxially drawn, while hollow fiber membranes are uniaxially drawn. This is generally performed by running the membranes over a pair of godets in which the latter godets are moving at a faster rate than the former godets. The draw down or elongation ratio is the ratio of the beginning length of the membrane to the final length of the membrane. Preferably the lower limit on the draw down or elongation ratio is about 1.05, more preferably 1.1. Preferably the upper limit on the draw down or elongation ratio is about 10. The membrane may be drawn in one or more stages with the options of using different temperatures, draw rates, and draw ratios in each stage. Line speeds are generally not critical and may vary significantly. Practical minimum preferred line speeds are at least about 10 feet/minute, more preferably at least about 30 feet/minute. Maximum preferred line speeds are less than about 2000 feet/minute, more preferably less than about 1000 feet/minute.

Optionally before or after leaching and/or drawing, the membranes may be annealed by exposing the membranes to elevated temperatures. The membranes may be annealed at temperatures above the glass transition temperature (Tg) of the syndiotactic polystyrene or the mixture and about 10° C. below the melting point of the syndiotactic polystyrene or depressed melting point of the mixture for a period of time between about 30 seconds and about 24 hours.

The transport rate through the anisotropic membrane is controlled by the resistance to transport created by the anisotropic character of the membrane. The thickness of the membrane is such that the membrane possesses adequate mechanical strength under use conditions and good separation characteristics. In the case of flat sheet membranes, the minimum thickness is preferably at least about 10 microns, more preferably at least about 15 microns; the maximum thickness is preferably less than about 1000 microns, more preferably less than about 800 microns. In the case of hollow fibers, the outer diameter of the membrane is preferably at least about 50 microns, more preferably at least about 70 microns; the outer diameter of the membrane is preferably less than about 5000 microns, more preferably less than about 4000 microns. The inside diameter of the hollow fiber membranes is preferably at least about 30 microns, more preferably at least about 40 microns; the inside diameter of the hollow fiber membranes is preferably less than about 4980 microns, more preferably less than about 3980 microns. The hollow fiber membrane thickness is preferably at least about 10 microns, more preferably at least about 15 microns; the membrane thickness is preferably less than about 1000 microns, more preferably less than about 800 microns.

The final membranes which are substantially free of solvent and nonsolvent preferably exhibit a glass transition temperature of at least about 80° C., more preferably of at least about 90° C., most preferably of at least about 100° C.

The membranes are fabricated into flat sheet, spiral, tubular, or hollow fiber devices by methods described in the art. Spiral wound, tubular, and hollow fiber devices are preferred. Tubesheets may be affixed to the membranes by techniques known in the art. The membrane is sealingly mounted in a pressure vessel in such a manner that the membrane separates the vessel into two fluid regions wherein fluid flow between the two regions is accomplished by fluid permeating through the membrane. Conventional membrane devices and fabrication procedures are well known in the art.

The membranes are useful in ultrafiltration, microfiltration, membrane distillation, and/or membrane stripping, and as supports for composite gas or liquid separation membranes.

In ultrafiltration or microfiltration, the membranes are used to recover or isolate solutes or particles from solutions. The membrane divides the separation chamber into two regions, a higher pressure side into which the feed solution is introduced and a lower pressure side. One side of the membrane is contacted with the feed solution under pressure, while a pressure differential is maintained across the membrane. To be useful, at least one of the particles or solutes of the solution is selectively retained on the high pressure side of the membrane while the remainder of the solution selectively passes through the membrane. Thus the membrane selectively "rejects" at least one of the particles or solutes in the solution, resulting in a retentate stream being withdrawn from the high pressure side of the membrane which is enriched or concentrated in the selectively rejected particle(s) or solute(s) and a filtrate stream being withdrawn from the low pressure side of the membrane which is depleted in the selectively rejected particle(s) or solute(s).

In membrane distillation or membrane stripping, the membranes are used to remove or recover more volatile components from solutions containing more volatile components and less volatile components. For example, membrane distillation and membrane stripping are useful for desalinating water and/or removing volatile organics from aqueous streams. The membrane divides the separation chamber into two regions. The feed stream containing more volatile and less volatile components is contacted with the non-permeate side of the membrane while contacting the permeate side of the membrane with a sweep gas such as nitrogen, carbon dioxide, air, or other inert gas, a vacuum, or a combination thereof, under conditions such that the more volatile components permeate through the membrane as a vapor. A chemical potential gradient is thus established across the membrane due to the difference in vapor pressure of the more volatile components across the membrane. When a sweep gas is used, in some embodiments it may be advantageous to maintain the pressure on the permeate side of the membrane at a pressure greater than the pressure on the non-permeate side of the membrane in order to prevent leakage of liquid from the non-permeate side of the membrane through defects in the membrane to the permeate side of the membrane. In membrane stripping, the more volatile components which permeate through the membrane as vapor are removed from the permeate side of the membrane as vapor; in membrane distillation, the more volatile components which permeate through the membrane as vapor are condensed on the permeate side of the membrane and removed as liquid.

The separation processes described hereinbefore should be carried out at pressures which do not adversely affect the membrane, that is, pressures which do not cause the membrane to mechanically fail. The pressure differential across the membrane is dependent upon the membrane characteristics, including pore size, porosity, and the thickness of the membrane, and in the case of hollow fiber membranes, the inside diameter. For the membranes of this invention, the pressure differential across the membrane is preferably between about 5 and about 500 psig, more preferably between about 10 and about 300 psig. The separation processes described hereinbefore should be carried out at temperatures which do not adversely affect membrane integrity. Under continuous operation, the operating temperature is preferably between about 0 and about 150° C., more preferably between about 15 and about 130° C.

The anisotropic microporous membranes of this invention may be characterized in a variety of ways, including porosity, mean pore size, maximum pore size, bubble point, gas flux, water flux, and molecular weight cut off. Such techniques are well known in the art for characterizing microporous membranes. See Robert Kesting, Synthetic Polymer Membranes, 2nd edition, John Wiley & Sons, New York, New York, 1985, pp. 43-64; Channing R. Robertson (Stanford University), Molecular and Macromolecular Sieving by Asymmetric Ultrafiltration Membranes, OWRT Report, NTIS No. PB85-1577661EAR, September 1984 and ASTM Test Method F316-86 and F317-72 (1982); the relevant portions incorporated herein by reference.

Porosity refers to the volumetric void volume of the membrane. Porosity may be determined gravimetrically from the density of the void-free polymer or from the differences between the wet and dry weights of the membrane. The membranes of this invention preferably have a porosity of at least about 5 percent, more preferably at least about 10 percent; the membranes of this invention preferably have a porosity of less than about 90 percent, more preferably of less than about 80 percent.

Pore size of the membrane may be estimated by several techniques including scanning electron microscopy, and/or measurements of bubble point, gas flux, water flux, and molecular weight cut off. The pore size of any given membrane is distributed over a range of pore sizes, which may be narrow or broad.

The bubble point pressure of a membrane is measured by mounting the membrane in a pressure cell with liquid in the pores of the membrane. The pressure of the cell is gradually increased until air bubbles permeate the membrane. Because larger pores become permeable at lower pressures, the first appearance of bubbles is indicative of the maximum pore size of the membrane If the number of pores which are permeable to air increases substantially with a small increase in pressure, a narrow pore size distribution is indicated. If the number of air-permeable pores increases gradually with increasing pressure, a broad pore size distribution is indicated. The relationship between pore size and bubble point pressure can be calculated from the equation: ##EQU1## where r is the pore radius

G is the surface tension (water/air), and

P is the pressure.

See ASTM F316-86, the relevant portions incorporated herein by reference.

The membranes of this invention useful for ultrafiltration preferably possess a maximum pore size which exhibits a bubble point in the range of about 90-100 psig or greater, more preferably in the range of about 180-190 psig or greater.

The mean pore size of the membranes of this invention useful for ultrafiltration is preferably between about 5 and about 1000 Angstroms, more preferably between about 10 and about 500 Angstroms; the maximum pore size of such membranes is preferably less than about 1000 Angstroms, more preferably less than about 800 Angstroms. The mean pore size of the membranes of this invention useful for microfiltration, membrane distillation, and/or membrane stripping is preferably between about 0.02 and about 10 microns, more preferably between about 0.05 and about 5 microns; the maximum pore size of such membranes is preferably less than about 10 microns, more preferably less than about 8 microns.

Gas flux is defined as ##EQU2## A standard gas flux unit is ##EQU3## abbreviated hereinafter as ##EQU4## where STP stands for standard temperature and pressure. The membranes of this invention preferably have a gas flux for nitrogen of at least about ##EQU5## more preferably of at least about ##EQU6##

Water flux is defined as ##EQU7## under given conditions of temperature and pressure. The water flux is commonly expressed in gallons per square foot of membrane area per day (GFD). The membranes of this invention preferably exhibit a water flux at about 50 psig and about 25° C. of at least about 1 GFD, more preferably of at least about 10 GFD. See ASTM 317-72 (1982).

SPECIFIC EMBODIMENTS

The following Examples are included for illustrative purposes only and are not intended to limit the scope of the invention or claims. All percentages are by weight unless otherwise indicated.

Example 1--Solubility Of Syndiotactic Polystyrene In Various Compounds

Mixtures consisting of approximately five weight percent polymer in various organic compounds are prepared in two dram-capacity glass vials that are subsequently sealed with aluminum foil liners. The mixtures are weighed to a precision of one milligram. The vials are placed in an air-circulating oven at about 125-140° C. Dissolution behavior is observed by transmitted light at close range from an AO universal microscope illuminator at progressively increasing temperatures until complete dissolution is observed, until the boiling point of the solvent is closely approached, or until 300° C. is reached (the approximate ceiling temperature of the syndiotactic polystyrene). The temperature is increased in about 25° C. increments. The mixtures are allowed to remain at a given temperature for at least about 30 minutes before the temperature is increased further. The hot mixtures are cooled to room temperature; their appearance is noted after they are allowed to stand undisturbed overnight at room temperature. The results are compiled in Table I. The polymer noted as "SYNDIO2" is a sample of syndiotactic polystyrene with a weight-average molecular weight of about 5.6×10⁵ daltons. The polymer noted as "SYNDIO" is a sample of syndiotactic polystyrene with a lower molecular weight.

                                      TABLE I                                      __________________________________________________________________________                                 APPROX.                                                   CONC.                B.P., TEMP.         APPEARANCE AT                  POLYMER                                                                               WGT. %                                                                              SOLVENT         DEG. C.                                                                              DEG. C.                                                                             SOLUBILITY                                                                              ROOM TEMP                      __________________________________________________________________________     SYNDIO 4.86 1-chloronaphthalene                                                                            250   211  Soluble  Firm hazy gel                  SYNDIO 5.08 1-cyclohexyl-2-pyrrolidinone                                                                   301   200  Soluble  Amber soft gel                 SYNDIO 4.95 1-phenyl-2-pyrrolidinone                                                                       345   200  Soluble  Opaque hard solid              SYNDIO 4.97 4-phenylphenol  321   211  Soluble  Opaque hard solid              SYNDIO 25.12                                                                               4-phenylphenol  321   221  Soluble  Opaque solid                   SYNDIO 5.16 benzil          347   211  Soluble  Yellow hard solid              SYNDIO 5.02 benzophenone    305   200  Soluble  Clear firm gel                 SYNDIO 4.70 caprolactam (epsilon)                                                                          271   211  Soluble  Opaque hard solid              SYNDIO 24.94                                                                               caprolactam (epsilon)                                                                          271   221  Soluble  Opaque hard solid              SYNDIO 5.29 diphenyl ether  259   211  Soluble  Firm hazy gel                  SYNDIO 5.35 diphenyl sulfone                                                                               379   231  Soluble  Opaque hard solid              SYNDIO 5.08 N,N-diphenylformamide                                                                          337   200  Soluble  Opaque hard solid              SYNDIO 5.21 o-dichlorobenzene                                                                              180   171  Soluble  Firm hazy gel                  SYNDIO 4.77 sulfolane       285   217  Not soluble                             SYNDIO 4.77 sulfolane       285   231  Soluble  Liquid slush                   SYNDIO2                                                                               5.09 1,2,3-trichlorobenzene                                                                         218   150  Soluble  White opaque hard solid        SYNDIO 4.72 1,2,4-trichlorobenzene                                                                         214   211  Soluble  Cloudy Soft gel                SYNDIO 5.19 1-benzyl-2-pyrrolidinone                                                                       420   211  Soluble  Amber clear firm gel           SYNDIO2                                                                               5.14 1,2,4-trichlorobenzene                                                                         214   136  Soluble  Cloudy stiff gel               SYNDIO2                                                                               5.58 1-benzyl-2-pyrrolidinone                                                                       420   224  Soluble  Amber hazy stiff gel           SYNDIO2                                                                               5.58 1-benzyl-2-pyrrolidinone                                                                       420   200  Partly soluble                          SYNDIO2                                                                               5.26 1-chloronaphthalene                                                                            258   136  Soluble  Hazy stiff gel                 SYNDIO2                                                                               5.16 1-chclohexyl-2-pyrrolidinone                                                                   301   136  Partly soluble                          SYNDIO2                                                                               5.16 1-cyclohexyl-2-pyrrolidinone                                                                   301   150  Soluble  Amber soft hazy gel            SYNDIO2                                                                               5.13 1-ethyl-2-pyrrolidinone                                                                        296   161  Soluble  Pale yellow opaque slush       SYNDIO2                                                                               5.15 1-methyl-2-pyrrolidinone                                                                       202   136  Soluble  Cloudy stiff gel               SYNDIO2                                                                               5.04 1-phenyl-2-pyrrolidinone                                                                       345   200  Soluble  Tan opaque hard solid          SYNDIO2                                                                               5.09 4-phenylphenol  321   225  Soluble  White opaque hard solid        SYNDIO2                                                                               5.09 4-phenylphenol  321   200  Almost soluble                          SYNDIO2                                                                               5.13 acetophenone    202   165  Soluble  Cloudy gel above solid         SYNDIO2                                                                               5.13 acetophenone    202   150  Almost soluble                          SYNDIO2                                                                               5.01 anisole         154   153  Soluble  Cloudy stiff gel               SYNDIO2                                                                               5.04 benzil          347   200  Soluble  Yellow opaque hard solid       SYNDIO2                                                                               5.04 benzil          347   150  Partially soluble                       SYNDIO2                                                                               5.05 benzophenone    305   188  Soluble  Clear stiff gel                SYNDIO2                                                                               5.05 benzophenone    305   165  Partly soluble                          SYNDIO2                                                                               5.67 benzyl alcohol  205   190  Almost soluble                          SYNDIO2                                                                               5.67 benzyl alcohol  205   204  Soluble  White opaque soft gel          SYNDIO2                                                                               5.12 butyl stearate  343   273  Soluble  White opaque fluid             SYNDIO2                                                                               5.12 butyl stearate  343   250  Partly soluble                          SYNDIO2                                                                               5.09 caprolactam (epsilon)                                                                          271   200  Soluble  Hard solid                     SYNDIO2                                                                               5.10 cyclohexanone   155   150  Soluble  Soft gel                       SYNDIO2                                                                               5.20 decahydronaphthalene (decalin)                                                                 190   188  Almost soluble                                                                          Moderately stiff slush         SYNDIO2                                                                               5.18 dimethyl phthalate                                                                             282   200  Partly soluble                          SYNDIO2                                                                               5.18 dimethyl phthalate                                                                             282   224  Soluble  White opaque slush             SYNDIO2                                                                               5.02 diphenyl ether  259   150  Soluble  Clear stiff gel                SYNDIO2                                                                               5.02 diphenyl ether  259   136  Partly soluble                          SYNDIO2                                                                               5.28 diphenyl sulfone                                                                               379   225  Soluble  Pale tan hard solid            SYNDIO2                                                                               5.19 ethyl benzoate  212   165  Almost soluble                          SYNDIO2                                                                               5.19 ethyl benzoate  212   188  Soluble  Stiff pale yellow hazy                                                         gel                            SYNDIO2                                                                               5.34 HB-40 (Monsanto)                                                                               325   151  Partly soluble                          SYNDIO2                                                                               5.34 HB-40 (Monsanto)                                                                               325   200  Soluble  Slightly hazy pale                                                             yellow                                                                         firm gel                       SYNDIO2                                                                               5.13 Mesitylene(1,3,5-trimethyl                                                                     163   161  Almost soluble                                                                          Stiff heterogeneous gel                    benzene)                                                           SYNDIO2                                                                               4.97 methyl benzoate 199   150  Soluble  Cloudy stiff gel               SYNDIO2                                                                               5.04 methyl laurate  262   250  Soluble  White opaque slush             SYNDIO2                                                                               5.04 methyl laurate  262   224  Almost soluble                          SYNDIO2                                                                               4.96 methyl myristate                                                                               323   241  Hazy & soluble??                        SYNDIO2                                                                               4.96 methyl myristate                                                                               323   255  Soluble  Opaque white slush             SYNDIO2                                                                               5.07 methyl salicylate                                                                              222   175  Soluble  Cloudy stiff gel               SYNDIO2                                                                               5.07 methyl salicylate                                                                              222   150  Not soluble                             SYNDIO2                                                                               5.06 methyl stearate 359   273  Soluble  Opaque solid                   SYNDIO2                                                                               5.06 methyl stearate 359   250  Partly soluble                          SYNDIO2                                                                               5.13 nitrobenzene    211   151  Soluble  Yellow cloudy firm gel         SYNDIO2                                                                               4.82 N,N-dimethylacetamide                                                                          165   165  Not Soluble                                                                             White slush                    SYNDIO2                                                                               5.04 N,N-diphenylformamide                                                                          337   225  Soluble  Brown hard solid               SYNDIO2                                                                               5.04 N,N-diphenylformamide                                                                          337   200  Almost soluble                          SYNDIO2                                                                               5.13 o-dichlorobenzene                                                                              180   150  Soluble  Cloudy stiff gel               SYNDIO2                                                                               5.13 o-dichlorobenzene                                                                              180   136  Partly soluble                          SYNDIO2                                                                               5.00 Santowax R (Monsanto)                                                                          364   166  Partially soluble                       SYNDIO2                                                                               5.00 Santowax R (Monsanto)                                                                          364   200  Soluble  Tan hard solid                 SYNDIO2                                                                               5.00 sulfolane       285   200  Not soluble                             SYNDIO2                                                                               5.00 sulfolane       285   249  Soluble  Light tan opaque firm                                                          gel                            SYNDIO2                                                                               5.00 sulfolane       285   225  Partially soluble                       SYNDIO2                                                                               5.27 tetrahydronaphthalene (tetralin)                                                               207   136  Soluble  Stiff hazy gel                 SYNDIO2                                                                               5.15 Therminol 66 (Monsanto)                                                                        340   200  Soluble  Slightly hazy pale yellow                                                      soft                                                                           gel                            SYNDIO2                                                                               5.15 Therminol 66 (Monsanto)                                                                        340   151  Partly soluble                          SYNDIO2                                                                               4.99 Therminol 75 (Monsanto)                                                                        385   200  Soluble  Yellow opaque firm                                                             solid/gel                      SYNDIO2                                                                               5.25 xylene          141   136  Soluble  Moderately stiff white                                                         opaque gel                     SYNDIO2                                                                               4.98 cyclohexylbenzene                                                                              239   181  Soluble  Cloudy firm gel                SYNDIO2                                                                               4.98 cyclohexylbenzene                                                                              239   158  Almost Soluble                          SYNDIO2                                                                               4.99 dicyclohexyl    227   200  Mostly soluble                          SYNDIO2                                                                               4.99 dicyclohexyl    227   225  Soluble  Homogeneous slush              SYNDIO2                                                                               4.98 methyl caproate 151   151  Not soluble                                                                             Clear liquid with solid                                                        polymer sediment               SYNDIO2                                                                               5.01 methyl caprylate                                                                               194   194  Not soluble                                                                             Milky liquid with solid                                                        sediment                       SYNDIO2                                                                               4.94 methyl enanthate                                                                               172   172  Not soluble                                                                             Water-clear liquid with                                                        polymer sediment               SYNDIO2                                                                               4.99 methyl valerate 128   128  Not soluble                                                                             Water-clear liquid with                                                        solid sediment                 SYNDIO2                                                                               4.96 n-butylbenzene  182   183  Mostly soluble                                                                          White opaque soft gel          SYNDIO2                                                                               4.96 n-butylbenzene  182   169  Heavily swollen                         SYNDIO2                                                                               4.96 n-butylbenzene  182   151  Not soluble                             SYNDIO2                                                                               5.00 n-propylbenzene 159   158  Soluble  White opaque firm gel          SYNDIO2                                                                               5.04 phenetole       169   128  Swollen                                 SYNDIO2                                                                               5.04 phenetole       169   150  Soluble  Hazy pink firm gel             SYNDIO2                                                                               5.35 phenol          182   155  Swollen                                 SYNDIO2                                                                               5.35 phenol          182   158  Almost soluble                          SYNDIO2                                                                               5.35 phenol          182   181  Soluble  Opaque white firm              __________________________________________________________________________                                                     gel                       

Example 2--Syndiotactic Polystyrene Flat Sheet Membrane Prepared Using A Protic Solvent

Syndiotactic polystyrene with a weight-average molecular weight of about 450,000 daltons is used to prepare a solution containing about 15.0 percent syndiotactic polystyrene and about 85.0 percent epsiloncaprolactam by weight. The mixture is dissolved by heating with mixing to about 244° C. A 0.015 inch drawing knife is used to cast a flat sheet membrane onto a Pyrex glass plate heated to about 214° C. The glass plate is immediately immersed in a water bath at about 21° C. The membrane is leached in water at room temperature overnight and dried. The membrane thickness is about 0.005 inches.

The membrane is mounted in an Amicon Model 8010 stirred cell using filter paper as a support. The effective membrane area is about 4.1 cm². The membrane is wetted with methanol and then water. The water flux at about 50 psi is determined to be about 0.022 ##EQU8## 

What is claimed is:
 1. A process for preparing a semi-permeable anisotropic syndiotactic polystyrene microporous membrane comprising the steps of: A. forming a mixture comprising:(i) syndiotactic polystyrene, and (ii) at least one protic solvent for the syndiotactic polystyrene; B. heating the mixture to a temperature under conditions such that a homogeneous fluid is formed which possesses sufficient viscosity to be formed into a membrane: C. extruding or casting the homogeneous fluid into a membrane; quenching or coagulating the membrane by passing the membrane through one or more zones under conditions such that the membrane solidifies: and E. simultaneously or consecutively leaching the membrane by passing the membrane through one or more zones under conditions such that at least a substantial portion of the solvent for the syndiotactic polystyrene is removed from the membrane:wherein the semi-permeable membrane so formed possesses anisotropic microporous structure.
 2. A process for preparing a semi-permeable anisotropic syndiotactic polystyrene microporous membrane comprising the steps of:A. forming a mixture comprising:(i) syndiotactic polystyrene, (ii) at least one protic solvent for the syndiotactic polystyrene, and (iii) at least one nonsolvent for the syndiotactic polystyrene; B. heating the mixture to a temperature under conditions such that a homogeneous fluid is formed which possesses sufficient viscosity to be formed into a membrane: C. extruding or casting the homogeneous fluid into a membrane; D. quenching or coagulating the membrane by passing the membrane through one or more zones under conditions such that the membrane solidifies: and E. simultaneously or consecutively leaching the membrane by passing the membrane through one or more zones under conditions such that at least a substantial portion of the solvent and nonsolvent for the syndiotactic polystyrene is removed from the membrane;wherein the semi-permeable membrane so formed possesses anisotropic microporous structure.
 3. The process of claim 1 or 2 which further comprises:F. drawing the membrane before, during, and/or after leaching at a temperature at or above ambient temperature and below the melting point of the syndiotactic polystyrene or the depressed melting point of the mixture to elongate the membrane and to induce orientation of the syndiotactic polystyrene in the membrane.
 4. The process of claim 3 wherein the amount of syndiotactic polystyrene in the mixture is between about 5 and about 90 weight percent.
 5. The process of claim 4 wherein the homogeneous fluid is extruded or cast at a temperature of between about 25 and about 325° C.
 6. The process of claim 5 wherein the membrane is quenched by:(i) passing the membrane through an air zone wherein the membrane begins to solidify: or (ii) passing the membrane through one or more liquid zones comprising a liquid which is a solvent for the syndiotactic polystyrene solvent and nonsolvent and which is not a solvent for the syndiotactic polystyrene, wherein the membrane is solidified and a portion of the syndiotactic polystyrene solvent and nonsolvent is removed: or (iii) passing the membrane through one or more liquid zones comprising a liquid which is not a solvent for the syndiotactic polystyrene solvent and nonsolvent and which is not a solvent for the syndiotactic polystyrene, wherein the membrane is solidified.
 7. The process of claim 6 wherein the temperature of the air quench zone is between about 0 and about 100° C.
 8. The process of claim 7 wherein the temperature of the liquid quench zone(s) is between about 0 and about 100° C.
 9. The process of claim 8 wherein the quench liquid is water, a lower alcohol, a halogenated hydrocarbon, or a perhalogenated carbon compound.
 10. The process of claim 9 wherein the membrane is drawn at a temperature of between about 10 and about 280° C.
 11. The process of claim 10 wherein the membrane is drawn to an elongation ratio of between about 1 and about
 10. 12. The process of claim 4 wherein the solvent comprises 4-phenylphenol, benzyl alcohol, bisphenol A, caprolactam, phenetole, phenol, or mixtures thereof.
 13. The process of claim 3 which furtherG. before or after leaching and/or drawing, the membrane is annealed by exposing the membrane to a temperature above the glass transition temperature of the syndiotactic polystyrene or the mixture and about 10° C. below the melting point of the syndiotactic polystyrene or the depressed melting point of the mixture for a period of time between about 30 seconds and about 24 hours. 